This invention relates to certain ester polyurethane foams that have been chemically modified, following which the foams have unexpectedly improved liquid absorption and wicking. The foams may be incorporated into articles used to wipe and absorb liquids, such as household cleaning sponges and mop heads.
Polyurethane foams are generally prepared by the reaction of an active hydrogen-containing compound (i.e., a polyol) and a polyisocyanate, in the presence of a blowing agent such as water, and usually a reaction catalyst and foam stabilizer. The cellular polymer structure of polyurethane foam has a skeletal framework of relatively heavy strands forming an outline for the cell structure. The skeletal framework strands are connected by very thin membranes, often called windows, which form the cell walls. In open-celled foams, some of the windows are open or torn in each cell, thus forming an interconnecting network open to fluid flow (liquid or gas). However, conventional polyurethane foams are not sufficiently porous or open-celled to allow significant fluid flow there through.
Reticulation relates to methods for removing or breaking the cell windows of polyurethane foams. Mechanical, chemical and thermal methods for reticulating foams are known. As one example, foam may be reticulated by melting the windows with a high temperature flame front or explosion, which still leaves the strand network intact. Alternatively, the cell windows may be etched away using the hydrolyzing action of water in the presence of an alkali metal hydroxide. See U.S. Pat. Nos. 3,125,542; 3,405,217; 3,423,338; 3,425,890 and 4,670,477 for descriptions of various reticulating methods for polyurethane foams.
Household cleaning sponges and mop heads most commonly are formed from cellulose. Paper pulp is the primary ingredient for cellulose sponges. The pulp is reacted with carbon disulfide to form a soluble cellulose xanthate compound. This compound is dissolved into a honey-like liquid viscose and mixed with reinforcing fibers to add strength to the pulp mixture. The cellulose is formed with a double cell structure to replicate natural sea sponges. Sodium sulfate crystals are added to the pulp, and this mixture is heated in a mold to melt the crystals. Heating regenerates the mix to pure cellulose and leaves the signature sponge holes where the crystals have melted away. Bleaching chemicals and humectants maintain the moisture level and color purity of the cellulose sponge. While the cellulose has good water absorption and wicking, it has lower wet integrity than other materials. Moreover, upon drying, the cellulose becomes hard and brittle such that it must be pre-wet before using for wiping.
Open celled ester and ether polyurethane foams have greater softness and flexibility than cellulose, and retain flexibility upon drying without humectants. As compared to cellulose, foams have greater wet strength, better wet integrity and exhibit less swelling when wet. Foams also can be foamed to have a double cell structure to more resemble natural sea sponges. Generally, polyurethane foams can be produced more cheaply than cellulose. However, polyurethane foams are hydrophobic, lacking good liquid absorption and wicking characteristics, which makes them less suitable for household sponges and mop heads. Even after the polyurethane foams are post-treated with surfactants in an attempt to improve water absorption and wicking, they still do not match the performance of cellulose for these properties.
Reticulated polyurethane foams have been used as components of filters. Such foams also have been suggested for use as components of household sponges, particularly for the abrasive surface presented by a reticulated foam. See U.S. Pat. Nos. 3,857,133 and 5,640,737. The art still seeks polyurethane foams suitable to replace cellulose materials as liquid absorbing and wicking components of household sponges and mop heads.
According to the invention, a hydrophilic ester polyurethane foam is made by first forming a cellular polyurethane foam that has a network of at least some strands and at least some cell windows by mixing together certain foam-forming components. Typically, the recipes for polyurethane foams are expressed in terms of parts by weight per 100 parts polyol. Thus, for each 100 parts by weight of a polyester polyol, the foam formulation according to the invention includes: from 20.0 to 62.0 parts by weight of an isocyanate; from 1.5 to 5.0 parts of a blowing agent, such as water; from 0.5 to 2.0 parts of a blow catalyst; from 0 to 0.3 parts of a gel catalyst, and from 1.0 to 3.0 parts of a stabilizing surfactant, such as a silicone surfactant. Other additives such as antimicrobial additives, double cell additives, dyes, pigments, colorants, crosslinking additives, fragrances, detergents and extenders may also be incorporated into the foam formulation.
After the foam forming components have been mixed together, the foam is permitted to rise and cure, preferably under atmospheric temperature and pressure. The resulting foam has pore sizes preferably in the range of from 70 to 130 pores per linear inch, most preferably 70 to 90 pores per linear inch, but may also have a double cell or sea sponge-like structure. The preferred double cell structure has a distribution of larger and medium sized cells scattered across a background of finer cells. The larger cells may range from 0.06 to 0.09 inches in diameter.
The cured foam is cut into slabs and then chemically treated by immersing the slab in a heated caustic bath for from three to fifteen minutes, preferably from six to ten minutes. One preferred caustic bath is a sodium hydroxide solution (from 5.0 to 10.0 percent, preferably 7.5% NaOH) that is heated to from 70xc2x0 F. to 160xc2x0 F., preferably from 120xc2x0 F. to 160xc2x0 F. The caustic solution etches away at least a portion of the cell windows within the foam cellular structure, leaving behind a hydrophilic ester polyurethane foam. The treated foam is compressed between calendaring rolls, and then rinsed thoroughly and oven dried.
After treating, the foam structure comprises a hydrophilic ester polyurethane foam with good wicking characteristics that will absorb water at a rate of at least 20 pounds of water per square foot per minute, preferably at least 25 pounds of water per square foot per minute. The foam also has greater water holding capacity and wet strength than cellulose. The hydrophilic ester polyurethane foam does not swell appreciably upon absorbing and retaining liquids and would make an ideal component of a household sponge or mop head.